The invention relates to an electronic control of a variable speed, automatic transmission mounted on a vehicle which is provided with a torque converter having a directly coupled clutch, in particular, to the control of an automatic lockup of the directly coupled clutch.
In a conventional practice, the lockup control of such transmission takes place in a manner such that the directly coupled clutch is actuated to couple the output shaft of the torque converter directly with an engine output shaft (lockup) automatically above a given vehicle speed in a particular speed stage, for example, a third or fourth speed stage and to deactuate the clutch to connect the input shaft of the torque converter with the engine output shaft at other times.
The torque converter has the feature that it enables a smooth starting, acceleration and speed change in accordance with the magnitude of a load while preventing or minimizing the drawbacks such as a knocking of an engine or an engine stop.
However, there is a problem that under a low load condition and a high rotation of the engine, a fluid coupling condition prevails to prevent a speed change from occurring, resulting in a degraded fuel cost which is attributable to a power loss due to slippage. One technique to overcome this problem is the use of the torque converter having the directly coupled clutch. By directly coupling the engine output shaft with the output shaft of the torque converter (lockup), the power loss is reduced, thus advantageously improving the fuel cost.
However, in the prior art practice, the lockup takes place only in response to a vehicle speed above a given value in the third speed stage or overdrive. Accordingly, the improvement achieved in respect of the fuel cost is only slight, and if an accelerator pedal is depressed to an increased degree, difficulties are experienced that the engine is subject to a knocking phenomenon or the torque converter fails to provide a desired torque amplification effect, thus resulting in a power performance which is less than desired. To obtain the respective advantages of the improved fuel cost through the lockup and the speed change through the torque converter, a complicated control is required as will be mentioned later. Since the prior art could only achieve a simple control of activating the lockup above a given vehicle speed in the third speed stage (for vehicles having up to the third speed stage) or overdrive (for vehicles having the fourth speed stage or overdrive range), so that the vehicle speed where the lockup is activated has been a compromise between the fuel cost and the power performance demand inasmuch as the power performance is greatly reduced if the lockup takes place at lower speed in order to improve the fuel cost preferentially while a reduction in the fuel cost is diminished during the normal running condition if the lockup takes place at a higher speed in order to improve the power performance preferentially.